CAPE CANAVERAL, Fla.—Two spacewalking astronauts tackled a backlog of outdoor work at the International Space Station on Tuesday.

It was the first spacewalk for Italy—a major contributor to the orbiting lab—as Luca Parmitano handled a variety of maintenance chores. He was accompanied by American Christopher Cassidy, a veteran spacewalker.

Cassidy encountered a stubborn bolt, eating up precious minutes, as he got started on the first of two planned spacewalks just a week apart. A slim gap of just one-eighth of an inch stalled the installation of a new space-to-ground radio transmitter. The old one failed in December.

"Nothing jumps out at me," Cassidy reported to Mission Control. "I can see a little wear on the bolt."

Finally, the former Navy SEAL managed to attach the transmitter. Mission Control said it appeared to be a tight fit.

It was smoother going for Parmitano as he collected science experiments for return to Earth later this year aboard a commercial SpaceX capsule.

The spacewalkers made up for lost time as they went through the hodgepodge of chores, removing a bad camera and relocating radiator grapple bars. Some of the work was done to make it easier to swap out bad parts if there's ever a breakdown.

They hustled through cable work in preparation for a new Russian lab due to arrive in December, and took pictures of the Alpha Magnetic Spectrometer, a $2 billion cosmic ray detector launched on NASA's next-to-last shuttle mission in 2011.

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Scientists noticed unusual discoloration on its radiators and requested photos of the instrument, which is anchored to the station.

Parmitano found it awkward to secure a protective cover over a docking port used by NASA's space shuttles until their retirement two years ago. The cover is meant to shield against micrometeorite strikes.

"It's kind of like when you're trying to make your bed by yourself. One side is a little bit shorter than the other," Parmitano said.

Before ending their six-hour excursion, the astronauts got a jump on work intended for a second spacewalk next Tuesday.

"A really great day," Mission Control radioed up.

NASA said the tasks had been piling up over the past couple of years. Managers wanted to wait until the to-do list was long before committing to the time-consuming spacewalks.

Parmitano, 36, a major in the Italian Air Force, arrived at the space station at the end of May for a six-month stay.

Cassidy, 43, will wrap up his half-year mission in September.

The rest of the space station crew—one American and three Russians—assisted the spacewalkers from inside.

This was the fourth spacewalk this year with five more planned, mostly by Russians. In all, 170 spacewalks have been performed over the past 15 years at the space station, totaling nearly 1,074 hours or 45 days.

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